Apneic ventilation using pressure-limited ventilation

Introduction

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Noninvasive ventilation (i.e. BiPAP) is arguably the most powerful approach to optimize oxygenation and ventilation before intubation, given its ability to provide 100% FiO2, PEEP, and ventilatory support.The only way to improve upon this is to extend the administration of positive pressure ventilation throughout sedation and paralysis, right up until the moment of intubation.Either a mechanical ventilator or some BiPAP machines can easily be set to deliver ventilator-triggered breaths after the patient becomes apneic.This is similar to manually bagging the patient, but using a machine improves precision and safety.Although unnecessary for most patients, apneic ventilation may be useful for patients at high risk of hypoxemia or acidosis.

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Nuts & bolts

Some newer BiPAP machines (e.g. Phillips BiPAP Vision and Phillips Respironics V60) can be set in a “spontaneous/timed” mode (S/T mode).As long as the patient is breathing at a rate higher than the set rate, S/T mode is identical to BiPAP.However, if the patient's respiratory rate drops below the set rate, machine-triggered breaths will be delivered (functioning identically to a traditional mechanical ventilator set on pressure-controlled ventilation).Apneic ventilation can also be performed by connecting a facemask to a traditional mechanical ventilator set to provide pressure-controlled ventilation.

The transition onto ventilator-supported breathing may be seamless.While the patient is breathing spontaneously, the machine can be set at a rate 5-10 breaths/minute below the patient's respiratory rate. This will have no effect until apnea occurs, when the machine will immediately begin providing pressure-controlled ventilation. At this point, the respiratory rate may be increased to the optimal rate for machine-triggered ventilation (e.g. 30 breaths/minute, as discussed below).

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Keep the airway open

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Whether performing apneic oxygenation or apneic ventilation, nothing works if the airway is occluded (e.g. due to the tongue falling backwards after paralysis).One advantage of apneic ventilation is that it provides a continuous monitor of whether the airway is open.The machine will display the tidal volumes that the patient is receiving.Following paralysis, the tidal volumes will fall, but they shouldn't fall to zero.If the tidal volumes fall very low, this suggests airway occlusion.Usually, patient positioning (i.e. ear to sternal notch) plus simple airway maneuvers (i.e. head tilt and chin lift) may open the airway.

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Machine settings to optimize oxygenation

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Physiology of recruitment: Understanding transpulmonary pressure

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When we think about using positive pressure to recruit the lungs, we generally think about PEEP.However, PEEP is only part of the story. For example, let's imagine a woman with severe ARDS who is placed on BiPAP 15cm/5cm. The pressure which is opening her alveoli is the transpulmonary pressure, which equals the difference between her alveolar pressure and pressure in her pleural cavity. This is equal to the positive pressure from BiPAP mask minus the pressure generated by her diaphragm (1):

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In this scenario, lung recruitment only occurs during inspiration, when her transpulmonary pressure is +30cm.During exhalation, her transpulmonary pressure is -3cm, which will de-recruit her lungs.Thus, the primary factors opening up her lungs are actually the peak pressure of the BiPAP machine (+15cm) and negative pressure produced by her diaphragm (-15cm), not the PEEP.

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The single most important number influencing her recruitment may be her mean transpulmonary pressure.For example, if she spends most of her time during inspiration, then her mean transpulmonary pressure will be closer to +30cm.Alternatively if she has a lower respiratory rate and is taking shorter breaths, then her mean transpulmonary pressure will be closer to -3cm.

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Now let's imagine that she is paralyzed prior to intubation.While apneic, her trans-pulmonary pressure may be stable at around 5cm.(Note that if she were obese, her diaphragm would compress her lungs during apnea, producing a transpulmonary pressure <5cm).We have just taken away her inspiration, which was recruiting her lungs with a pressure of +30cm.Without these bursts of pressure during inspiration, her lungs may collapse before intubation.

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How to set the BiPAP machine during apnea to optimize trans-pulmonary pressure

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The transpulmonary pressure may be approximated using the equations below (2).The driving pressure is equal to the Peak Pressure minus the PEEP, which is the pressure differential that drives gas into the lungs with each breath (thus determining how large each tidal volume is).

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The peak pressure must be limited to avoid gastric insufflation.This establishes a tradeoff between the PEEP and the driving pressure: the higher the driving pressure is, the lower the PEEP must be.

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For a patient with severe hypoxemia, we generally want to maximize the transpulmonary pressure even if this occurs at the cost of reducing ventilation.This may be achieved by increasing the PEEP.

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The other approach to improve transpulmonary pressure is to increase the percent of the time that the patient will spend during inhalation.This may be done either by increasing the respiratory rate, or by increasing the inspiratory time of each breath.The best way to do this is to increase the respiratory rate, because this will simultaneously improve oxygenation and ventilation.A respiratory rate of 30 breaths/minute with a one-second inspiratory time will cause half of the time to be spent during inspiration (Inspiration : Expiration ratio of 1:1). This is the highest fraction achievable with a Respironics BiPAP machine (3).

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Therefore:

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Machine settings to optimize ventilation

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Less commonly than hypoxemia, we encounter patients with severe metabolic acidosis and a compensatory respiratory alkalosis (e.g. diabetic ketoacidosis or salicylate intoxication).These patients are at risk of acidosis in the peri-intubation period because we are taking away their compensatory respiratory alkalosisand potentially replacing it with a respiratory acidosis.Intubation of such patients should be avoided if possible (discussed previously on the post about DKA).However, sometimes it is unavoidable.In such situations, all efforts must be made to maintain the PaCO2 as low as possible throughout the peri-intubation period.

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Often such patients have normal lungs, in which case the ventilator settings can be set to maximize ventilation.This may be achieved by maximizing the driving pressure and decreasing the PEEP to zero (4).

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Therefore:

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Safety and maximal peak pressure

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Interposing ventilation between paralysis and intubation is controversial.Some would argue that true rapid sequence intubation (RSI) involves no ventilation, with any ventilation increasing the aspiration risk.However, for a patient at high risk of desaturation it may be safer to perform apneic ventilation up-front in a controlled fashion, thereby extending the safe apnea time and increasing the likelihood of first-pass success.Providing no ventilation up-front often results in the patient desaturating and requiring urgent manual bagging.With eitherapproach, apneic ventilation occurs; it's simply a matter of timing and control.Pressure controlled ventilation has been shown to result in lower peak pressures compared to manual ventilation, implying greater safety (Goedecke 2004).

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The maximal pressure that may be safely applied without insufflating the stomach and causing regurgitation is unclear.Previous studies based on auscultating the stomach have suggested that pressures <20-25cm are safe.A recent prospective, randomized, double-blind study using ultrasonography to evaluate gastric insufflation found that 15cm provided the ideal balance between avoiding gastric insufflation and providing adequate ventilation (Bouvet 2013)(6).The safety and efficacy of apneic ventilation using pressure-limited ventilation with a peak pressure of 15cm has been validated in elective surgical patients (Joffe 2010).

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Advantages of pressure-limited ventilation

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In general, mechanical ventilation can either be volume-limitedor pressure-limited.With volume-limited ventilation, the tidal volume is set by the practitioner and the pressure will vary depending on the lung compliance.Alternatively, with pressure-limited ventilation, the peak pressure is set by the practitioner and the volume will vary depending on the lung compliance.Similar results can generally be achieved with either mode.However, for the purpose of apneic ventilation, pressure-limited ventilation has some unique advantages:

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Pressure-limited ventilation guarantees a safe inspiratory pressure.

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Using a pressure-limited mode, as long as the inspiratory pressure is set at a safe level (i.e., 15 cm), this will guarantee that unsafe levels of pressure never occur.Alternatively, if volume-limited ventilation is used, then high pressures can occur.Volume-limited ventilation requires close monitoring with adjustment of tidal volumes to avoid dangerous pressures, a complex task requiring ongoing attention.

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The trade-off here is that the tidal volume is not guaranteed, so some patients may receive low tidal volumes.Choosing pressure-limited ventilation thus prioritizes safety over efficacy.Given that a normal minute ventilation is not mandatory during apnea, and that aspiration can be a major problem, this is a sensible trade-off.

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Pressure-limited ventilation maximizes the efficiency of inspiration.

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Compared to volume-limited ventilation, pressure-limited ventilation will maximize the tidal volume for a given peak pressure (e.g. 15 cm).With volume-limited ventilation, the airway pressure only reaches the peak pressure at the very last moment (figure below).In contrast, with pressure-limited ventilation the airway pressure is equal to the peak pressure throughout inspiration.Since pressure-limited ventilation maximizes the driving pressure throughout the entire breath, it will achieve a higher tidal volume compared to volume-limited ventilation with the same peak pressure (5). Seet 2009 confirmed this, demonstrating that pressure-limited ventilation achieved the same tidal volume as volume-limited ventilation despite using a lower peak pressure.00

When should apneic ventilation be considered?

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Although apneic ventilation is a useful tool for the toolbox, it is only occasionally needed.Patients who may benefit the most include those with profound hypoxemia, severe metabolic acidosis, or morbid obesity.

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For a patient requiring intubation who is already on a BiPAP machine capable of delivering apneic ventilation, this should be considered.The primary drawback of apneic ventilation is the logistics of connecting the patient to noninvasive ventilation.In this situation, apneic ventilation can be accomplished by pushing a few buttons on the BiPAP machine.

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Ventilatory support up until the moment of intubation may be easily provided using more sophisticated BiPAP machines (which can be set to provide backup respirations as soon as the patient stops breathing) or a mechanical ventilator.

Continuing ventilator support until intubation improves oxygenation and ventilation during paralysis. This may be useful for patients at high risk of hypoxemia (due to lung collapse) or acidosis (due to metabolic acidosis).

Expert mask-ventilation technique is critical to maintain an open airway after paralysis.

Use of pressure-limited ventilation during apnea guarantees avoidance of high inspiratory pressures that could cause gastric distension and aspiration.

Patients with hypoxemia may benefit more from PEEP, whereas patients at risk from hypercapnia may benefit more from higher driving pressures. The following is a rough guide to setting up apneic ventilation for different types of patients:

Disclosures: I have no conflicts of interest nor any relationship with drug or device manufacturers.

Notes

(1) Please note that in general exhalation is usually a passive process with no diaphragmatic activity.However, in the setting of respiratory distress it may become an active process.Also note that this patient's diaphragmatic pressures cannot be measured, and these are simply what I am imagining they might be. Finally, note that this is a simplification which assumes zero airway resistance (such that alveolar pressure is equal to airway pressure).

(2) Note again that this ignores any passive pressure exerted by the diaphragm to compress the lungs during apnea, for example due to pregnancy or obesity.

(3) In theory, the respiratory rate could even be increased further, to achieve inverse ratio ventilation (inspiratory time > expiratory time), similar to the concept of airway pressure release ventilation (APRV).However, the Respironics BiPAP machines will not allow inverse ratio ventilation (in most routine situations, inverse-ratio ventilation would result from an operator error and would be undesirable). Using a complete mechanical ventilator, inverse ratio ventilation could be used to further improve oxygenation (although this benefit might occur at the cost of impaired ventilation).

(4) The ideal respiratory rate to maximize ventilation is unclear.Minute ventilation is proportional to respiratory rate, so in general increasing respiratory rate is beneficial.However, if respiratory rate is increased too much, then there will be insufficient time for the lungs to fill and empty with each breath, causing the tidal volume to fall.A respiratory rate of about 30 breaths/minute may be a reasonable compromise.

Note also that typical recommendations for respiratory rate during manual bagging (i.e. limiting the respiratory rate to 10-12 breaths/minute) are designed to take into account that manual bagging is a volume-limitedprocess with a risk of progressive accumulation of excess gas in the chest (which may cause the intrathoracic pressure to spiral out of control).Using pressure-limited mechanical ventilation, it is impossible for this to happen.

(5) For intubated patients, minimizing the peak pressure doesn't matter so much (because the plateau pressure is more important than the peak pressure).However, for mask ventilation of non-intubated patients, the peak pressure is critically important to avoid gastric insufflation.Therefore, the lower peak pressures achievable with pressure-limited ventilation becomes a significant advantage.

(6) Note that there has been no evidence directly linking the level of pressure to clinical aspiration.It is possible that a minimal amount of gastric insufflation (e.g. as detected by ultrasound) may be clinically irrelevant.Thus, it is possible that higher pressures (e.g. 20-25cm) are safe.Unfortunately it is unlikely that a study relating pressure to clinical aspiration will be done, because this would require a very large sample size.

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Comment Here

A few things, my friend:1. The title of the post is entirely correct, but may be misleading. Technically this is ventilation during the apneic period–which of course shortens nicely to apneic ventilation. But the latter term seems more in keeping what something like High-Flow NC generates.

2. There is no advantage to a resp rate of 30 in the max. oxygenation. There is no increase in oxygenation from increased alveolar ventilation beyond a threshold. With 100% fiO2, that threshold is probably 1-2 breaths per minute. All the additional breaths simply increase risk without benefit. 4 breaths during the 60 seconds of apnea is probably the best balance of risk/benefit and what we advocate for bagging during apnea.

3. Prob safer and more in keeping with airway management skills to switch BIPAP mask to the mask of the BVM (continuing with same settings above) during apnea. This encourages proper airway opening (required with either mask) and allows vent-as-bag during failed intubation.

Thanks for your comments. #1 – Yep, Apneic ventilation is a bit of an oxymoron. #2 – Agree in most cases the respiratory rate doesn’t affect oxygenation much as long as the PEEP is high (i.e. 10 cm). However, for the most severe ARDS patients, increasing the respiratory rate will increase the percent of time spent in inspiration with a marginal increase in the mean airway pressure. For example, with a PEEP of 10, peak pressure of 15, and inspiratory time of one second, increasing the respiratory rate from 5 breaths/min to 30 breaths/min increases the mean airway pressure from 10.4cm to 12.5cm. Not a big difference, but either pressure may be low for really bad ARDS. I think pressure-limited ventilation with a peak pressure of 15cm ought to be fairly safe. Regardless of the respiratory rate, the peak pressure will never go above 15cm (for example, even if there is tons of autoPEEP, the tidal volume will drop but the peak pressure will never rise above 15cm). If anything, 15cm may be on the conservative/safe side. According to a landmark paper by Scott Weingart (Annals of EM 2012 59:165), inspiratory pressures >25cm are needed to overwhelm the esophageal sphincter… Read more »